Is there a way to increase the storage?

[jiapei100]

Yes you can boot from the SD card aswell.
In fact u-boot first tries booting of the SD card and only if there is no (bootable) SD card
will it try to boot from eMMC (to the best of my knowledge).

That's great... Is there any recommended operating system for pinephone? I noticed postmarketOS and UBports are 2 operating systems for pinephone. 

I wonder is Manjaro 20.04 or Ubuntu 20.04 have a version for pinephone instead of pinebook?

Cheers
Pei